The present invention relates to a coating liquid for the formation of porous silica coating, which enables forming an insulating film having a void content of as large as at least 30% and having excellent adherence to a substrate surface excellent, mechanical strength, chemical resistance (e.g., alkali resistance) and crack resistance and which enables the flattening of irregularities of a substrate surface to a high degree. Further, the present invention relates to a substrate provided with the above porous silica coating, a short fiber silica and a process for producing such a silica.
The silica coating is used in the following fields.
(1) Semiconductor Device:
In semiconductor devices, an insulating coating is provided, for insulation, between a semiconductor substrate and a metal wiring layer such as an aluminum wiring layer or between metal wiring layers. Further, in semiconductor devices, various elements such as a PN junction semiconductor, a condenser element and a resistor element superimposed on a semiconductor substrate are covered with an insulating coating in order to protect them. When, for example, a metal wiring layer is disposed on the semiconductor substrate, the surface of the semiconductor substrate becomes irregular due to the metal wiring layer. Formation of a further metal wiring layer or the like on the irregular surface may cause a disconnection of wiring because of the level difference attributed to the irregularity. Consequently, in semiconductor devices, the silica coating is used as an insulating coating capable of effectively flattening the irregular surface brought about by the above metal wiring layer and other various elements.
(2) Liquid Crystal Display:
For example, the matrix type color liquid crystal display is fitted with a liquid crystal display cell comprising an electrode plate, a counter electrode plate and a liquid crystal layer interposed between the electrode plate and the counter electrode plate. The electrode comprises a glass plate and, superimposed thereon, a picture element electrode composed of, for example, TFT (thin film transistor). The counter electrode comprises a glass plate and, superimposed thereon, a color filter and transparent electrodes in this order.
In the liquid crystal display cell used in the liquid crystal display, the picture element electrode protrudes on the electrode plate and the color filter protrudes on the counter electrode plate, so that the respective surfaces of these electrode plates have level differences. The level differences of the electrode surfaces cause a cell gap to be nonuniform, so that the alignment of liquid crystal material sealed inside the liquid crystal display cell tends to be disordered and the displayed image tends to suffer from picture disorder such as color shade. Therefore, it was proposed to provide the silica coating on the picture element electrode of the electrode plate and on the color filter of the counter electrode plate, thereby flattening irregular surfaces attributed to the picture element electrode and the color filter (see Japanese Patent Laid-open Publication No. 2(1990)-242226).
(3) Photomask with Phase Shifter:
The method is known in which the silica coating is provided as a phase shifter disposed for deviating the phase of irradiation light on a photomask so that a high-resolution irregular pattern is formed on a substrate by lithography, thereby enhancing the resolution of irregular pattern formed on the substrate (Nikkei Microdevice No. 71, 52 58, (5), 1991).
The silica coating employed in the above fields is generally formed on the substrate by the vapor phase growing method such as the CVD process or sputtering process or the coating method in which a coating film is formed from a coating liquid for forming silica coating. However, the vapor phase growing method such as the CVD process has drawbacks in that workload is heavy and large facilities are required and in that it is difficult to flatten the irregular surface of the substrate.
By contrast, the coating method is widely performed because large facilities are not required and because the flattening of the irregular surface can easily be carried out.
In the formation of the silica coating according to the above coating method, use is made of a coating liquid for forming silica coating which contains a polycondensate of a partial hydrolyzate of alkoxysilane as a film forming component. However, the formation of the coating film from the above coating liquid for forming silica coating has a drawback in that, during the stage of polycondensation of the partial hydrolyzate of alkoxysilane, silanol groups mutually induce a dehydration reaction at segments other than the terminals of the condensate so that the crosslinking of the condensate is advanced. This, in the formation of the silica coating, leads to an intense film shrinkage stress, thereby causing a film cracking with the result that it is difficult to obtain a silica coating having excellent crack resistance.
By contrast, the coating liquid for forming silica coating which contains fine particles of silica was proposed (see, for example, Japanese Patent Laid-open Publication No. 5(1993)-263045). It is known that the formation of the silica coating from the above coating liquid improves the crack resistance of the silica coating to a certain degree. In this coating liquid for forming silica coating, spherical silica particles obtained by hydrolyzing an alkoxysilane are used as the fine particles of silica. However, unreacted alkoxy groups remain in such fine particles of silica, so that, at the time of film formation, the alkoxy groups are oxidized so as to change to silanol groups. The silanol groups are likely to undergo a dehydration reaction to thereby advance the crosslinking of the condensate with the result that the crack resistance of the coating film is not satisfactory. A further problem is that because the fine particles of silica are spherical, the bonding strength between the fine particles of silica is unsatisfactory, thereby resulting in unsatisfactory film strength.
The inventors have conducted extensive studies on the basis of the above views. As a result, it has been found that
fine particles of silica prepared under specified conditions scarcely contain unreacted alkoxy groups and are in the form of short fibers;
the silica coating film formed from the coating liquid which contains the above short fiber silica is porous and maintains a desirable film strength; and
a coated substrate whose performance is superior to that of the prior art can be obtained by the use of the above coating liquid which contains the short fiber silica. The present invention has been completed on the basis of these findings.
The present invention has been made with a view toward solving the above problem of the prior art. An object of the present invention is to provide a coating liquid for forming porous silica coating, which enables the formation of an insulating film having excellent in adherence to a substrate surface, excellent mechanical strength, chemical resistance (e.g., alkali resistance) and crack resistance and which enables the flattening of irregularities of a substrate surface to a high degree. Another object of the present invention is to provide a substrate furnished with the above porous silica coating having excellent properties.
The coating liquid for forming porous silica coating according to the present invention comprises a product of reaction between:
a short fiber silica, and
a hydrolyzate of an alkoxysilane represented by the below indicated general formula [1] or a halogenated silane represented by the below indicated general formula [2]:
XnSi(OR)4-nxe2x80x83xe2x80x83[1],
XnSiXxe2x80x24-nxe2x80x83xe2x80x83[2],
wherein X represents a hydrogen atom, a fluorine atom, an alkyl group having 1 to 8 carbon atoms, an aryl group or a vinyl group; R represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group or a vinyl group; Xxe2x80x2 represents a chlorine atom or a bromine atom; and n is an integer of 0 to 3.
The above short fiber silica is preferably one obtained by hydrolyzing at least one alkoxysilane represented by the above general formula (11 and, thereafter, subjecting the hydrolyzate to a hydrothermal treatment at 250xc2x0 C. or higher temperature.
The coated substrate of the present invention comprises a porous silica coating film formed from the above coating liquid for forming porous silica coating. The short fiber silica of the present invention has an average diameter (D) of 10 to 30 nm, a length (L) of 30 to 100 nm and an aspect ratio (L/D) of 3 to 10.
This short fiber silica can be produced by a process comprising the steps of:
adding a catalyst such as ammonia to a solution of a mixture of water, an organic solvent and at least one alkoxysilane represented by the above general formula [1] to thereby conduct a hydrolysis of the alkoxysilane so that fine particles of silica having a particle size of 10 to 30 nm are produced;
removing any unreacted alkoxysilane, the organic solvent and the catalyst from the thus obtained reaction mixture solution to thereby obtain a water dispersion of fine particles of silica;
regulating so that the water dispersion has a fine silica particle solid content of 0.1 to 5% by weight and a catalyst concentration of 50 to 400 ppm in the term of ammonia; and
subjecting the water dispersion to a hydrothermal treatment at 250xc2x0 C. or higher temperature.